Translin. TRAX Complex (C3PO), a Novel Ribonuclease for the Degradation of Small RNAs.

C3PO is a heteromeric complex of two evolutionarily related proteins, translin and TRAX. C3PO was recently discovered to possess ribonuclease activity and promote the activation of RNA-induced silencing complex (RISC) in Drosophila and human by degrading the passenger strand fragments of duplex siRNA. C3PO is also involved in tRNA processing in the filamentous fungus Neurospora crassa and degrades the 5'-leader fragment that is cleaved off the precursor tRNA (pre-tRNA) by RNase P. Structural and biochemical studies revealed that active C3PO adopts a football-shaped hetero-octamer configuration containing six translin and two TRAX subunits. The RNA binding and catalytic residues are located at the interface between TRAX and translin subunits facing the hollow interior of C3PO octamer, indicating that C3PO binds and cleaves RNA inside its largely closed barrel. The ability of translin and TRAX to form complexes of different oligomeric states, such as tetramers, hexamers, and octamers, suggests a novel mechanism for the recruitment of RNA substrate and the assembly of active C3PO. Further mechanistic and functional studies are required to fully understand the role of C3PO in a number of biological processes.